Add SpdyString alias for std::string in net/spdy.

net/spdy/hpack/hpack_huffman_table.cc

Index: net/spdy/hpack/hpack_huffman_table.cc
diff --git a/net/spdy/hpack/hpack_huffman_table.cc b/net/spdy/hpack/hpack_huffman_table.cc
index 2a25c45ea0ade215dfe63e8acd8b80ebb0231eb8..a398d263a733a011f964f66c28e6430d18bb1824 100644
--- a/net/spdy/hpack/hpack_huffman_table.cc
+++ b/net/spdy/hpack/hpack_huffman_table.cc
@@ -16,8 +16,6 @@
 
 namespace net {
 
-using std::string;
-
 namespace {
 
 // How many bits to index in the root decode table.
@@ -266,7 +264,7 @@ size_t HpackHuffmanTable::EncodedSize(SpdyStringPiece in) const {
 }
 
 bool HpackHuffmanTable::GenericDecodeString(HpackInputStream* in,
-                                            string* out) const {
+                                            SpdyString* out) const {
   // Number of decode iterations required for a 32-bit code.
   const int kDecodeIterations = static_cast<int>(
       std::ceil((32.f - kDecodeTableRootBits) / kDecodeTableBranchBits));